integrated optical circuits are the basis of a new emerging field. Light is guided through integrated optical circuits by means of planar optical waveguides which are constructed from amorphous or crystalline dielectric materials that are transparent to the energy being transmitted. Planar optical waveguides as herein referred to are films or layers of such materials having a thickness equal to about the wavelength of the light or energy transmitted and a predetermined desired width. Ordinarily waveguides are termed planar when the width thereof is substantially greater than the thickness, while waveguides having a smaller width to thickness ratio are termed rectangular waveguides. For the purposes of the present invention, however, the term planar waveguide will be used to denote all non-circular waveguides regardless of the width to thickness ratios thereof.
It is well known to one skilled in the art that light can be caused to propagate along a transparent member which has a higher refractive index than its surroundings. Planar optical waveguides are useful in integrated optical circuits for computers or for coupling, demodulation, or other purposes in optical communications systems. The advantages of integrated optical circuits over electrical circuits is speed of signal transmission, ruggedness, long life, and the like. Waveguides produced for these purposes must avoid excessive attenuation of the transmitted light to be effective. Further, to be an effective transmitting medium within an integrated optical circuit, a planar waveguide should not only transmit light without excessive attenuation, but also should not cause excessive dispersion of the transmitted light and should allow only preselected modes of light to propagate along the waveguide. To the extent that planar waveguides have heretofore been fabricated, they have not been found to permit high transmission.
Some operational theories and other pertinent information concerning planar optical waveguides may be found in the publication "Evanescent Field Coupling into a Thin-Film Waveguide" by J. E. Midwinter, IEEE Journal of Quantum Electronics, Vol. QE-6, No. 10, October, 1970, pages 583-590; "Light Waves in Thin Films and Integrated Optics" by P. K. Tien, Applied Optics, Vol. 10, No. 11, November, 1971, pages 2395-2413; and "Dielectric Rectangular Waveguide and Directional Coupler for Integrated Optics" by E. A. J. Marcatili, The Bell System Technical Journal, Vol. 48, No. 7, September 1969, pages 2071-2102.
The propagation of light waves is governed by the same laws of physics that govern microwave propagation and, therefore, can also be studied in terms of modes. Since each mode of light traveling along a planar waveguide propagates at its own inherent velocity, it can be shown that information initially supplied to all modes will be dispersed after traveling a given length of waveguide due to different propagation velocities. Producing a satisfactory planar optical waveguide has been one of the more difficult problems in the development of effective integrated optical circuits.